1. Field of the Invention
This invention relates to a circuit board which is advantageous for forming a multiple-layered wiring pattern.
2. Brief Description of the Prior Art
With electronic devices being progressively made much smaller and thinner, there are increasingly strong demands for a circuit board which is designed to have a much smaller and lighter construction so that the circuit board will be suited to be used for those electronic devices which are being made much smaller and thinner.
One such representative example is a circuit board called a "multi-chip module (MCM)". In the MCM, a plurality of bare ICs are mounted on a multiple-layered circuit board and then mounted, as a single component part, on a circuit board acting as a mother board.
It is particularly strongly demanded of such a MCM circuit board that wiring is designed very small and arranged at a high density. At the same time, it is strongly demanded that the multiple-layered circuit board be designed thinner and manufactured at a lower cost.
One of the hindrances, which makes it difficult to meet such a demand, resides in the conventional techniques in which layers of the wiring patterns are inter-connected by means of a through-hole connection method. In this through-hole connection method, an insulative board is provided with very tiny connection holes which are formed all the way through the wiring patterns by drilling, laser or the like, and an inner wall surface of each connection hole is plated with a conductive layer (namely, the inner wall surface of each connection hole is subjected to a so-called through-hole plating). This conventional method has such shortcomings that when holes are bored in the insulative board by drilling or other means, burrs and metal powder tend to be produced, thus tending to degrade the quality and reliability of the through-hole plating.
Particularly, with the wiring patterns being made much smaller in design, it became increasingly technically difficult to make an accurate drilling or the like with respect to the insulative board without accidentally cutting off the wiring patterns.
Furthermore, the conventional techniques often encounter such problems that the areas for mounting the electronic parts and the areas for wiring are obliged to be limited because of a provision of the through-holes and due to installation of the wiring.
In order to cope with such shortcomings and problems, one approach is made in which the through-holes formed in the wiring patterns are filled with resin, and then the wiring and electrodes for mounting parts are applied to the top thereof. These through-holes are called "blind barrier holes" or "inner via hole", and are already utilized in actual practice.
However, they have problems not only in the area of techniques but also of cost. Accordingly, it is demanded to realize that wiring can be made at a very small pitch, and a circuit board, which is substantially of the blind barrier hole type or the inner via hole type, can be manufactured at a low cost.